Nuclear Magnetic Resonance (NMR) tools used for well-logging or downhole fluid characterization measure the response of nuclear spins in formation fluids to applied magnetic fields. Downhole NMR tools typically have a permanent magnet that produces a static magnetic field at a desired test location (e.g., where the fluid is located). The static magnetic field produces a magnetization in the fluid. The magnetization is aligned along the direction of the static field. The magnitude of the induced magnetization is proportional to the magnitude of the static field. A transmitter antenna produces a time-dependent radio frequency magnetic field that has a component perpendicular to the direction of the static field. The NMR resonance condition is satisfied when the radio frequency is equal to the Larmor frequency, which is proportional to the magnitude of the static magnetic field. The radio frequency magnetic field produces a torque on the magnetization vector that causes it to rotate about the axis of the applied radio frequency field. The rotation results in the magnetization vector developing a component perpendicular to the direction of the static magnetic field. This causes the magnetization vector to precess around the static field at the Larmor frequency. At resonance between the Larmor and transmitter frequencies, the magnetization is tipped to the transverse plane (i.e., a plane normal to static magnetic field vector). A series of radio frequency pulses are applied to generate spin echoes that are measured with the antenna.
NMR measurements can be used to estimate, among other things, formation porosity and formation permeability. For example, the area under the curve of a T2 distribution for a NMR measurement equals the NMR porosity. The T2 distribution also resembles the pore size distribution in water-saturated rocks. The raw reported porosity is provided by the ratio of the initial amplitude of the raw decay and the tool response in a water tank. This porosity is independent of the lithology of the rock matrix.
The NMR estimate of permeability is based on a theoretical model such as the Free-fluid (Coates) model or the average-T2 model. The Free-fluid model can be applied in formations containing water and/or hydrocarbons, while the average-T2 model can be applied to pore systems containing water only. Measurements on core samples are generally required to refine and customize those models for local use. The NMR permeability tends to increase with both increasing porosity and increasing pore size.
Crude oil properties such as viscosity, molecular composition, gas-oil ratio, and SARA (saturates, aromatics, resins, asphaltenes) fractions are crucial parameters for evaluating, for example, reservoir quality, producibility, and compartmentalization. Physical and empirical model-based equations have been developed which relate the properties of crude oils to Nuclear Magnetic Resonance (NMR) measurements. NMR response of fluids provides a link between microscopic molecular motions and macroscopic properties such as viscosity and composition. The relationship between viscosity and relaxation time of pure fluids was established by the phenomenological relaxation theory of Bloembergen, Purcell, and Pound (BPP). Brown studied proton relaxation in a suite of crude oils with various compositions and viscosities. The viscosities of the samples varied from about 0.5 to 400 cp. He found that the relaxation times showed an inverse dependence on viscosity over the entire range. Since the early work of Brown, several physical and empirical models have been proposed that relate crude oil properties to NMR response. Understanding of molecular dynamics in alkane mixture resulted in the establishment of a scale-law theory to relate NMR diffusion and relaxation properties to the molecular composition of crude oils. There are also other database approaches such as, for example, Artificial Neural Networks (ANN) and Radial Basis Function.
Characterization of reservoir fluids is crucial for several aspects of reservoir development and management. For example, fluid properties such as viscosity and molecular composition are used to calculate flow rates and sweep efficiencies of secondary and tertiary recoveries. Gas-oil ratio (GOR) of reservoir fluids is an important parameter for material selection of well completion and design of surface facilities. Asphaltene and wax concentrations are key considerations for flow assurance in completions, pipelines, and surface facilities. Estimation of fluid properties at different depths in a reservoir provides indications of compositional grading and compartmentalization within the reservoir. It is useful to obtain fluid properties from measurements such as NMR well-logging which can be performed at downhole temperature and pressure conditions.
Borehole images allow one to interpret the rock record for oil and gas exploration. In addition to identifying fractures and faults, borehole imaging tools are used for a variety of other applications such as sequence stratigraphy, facies reconstruction, stratigraphy, and diagenetic analysis. They can be used in a wide variety of geological and drilling environments, providing high resolution borehole images of rock and fluid properties in formations ranging from fractured carbonates to soft, thinly laminated sand/shale sequences. These tools produce high resolution and often nearly complete borehole coverage, which may be interpreted at an interactive graphics workstation.
NMR well-logging tools differ from those commonly used in the medical field in many respects. Obviously the operating environment for a downhole tool is much harsher than the laboratory setting of an imaging facility. In addition, a downhole NMR is configured “inside-out” relative to a typical “closed” medical NMR device. That is, medical devices usually look inward to their targeted area, whereas downhole NMR devices look outward into the surrounding formation.